fix(schema): allow empty array for required field with minItems

[spiderocious]

Fixes a bug where a required array property fails validation when given an
empty array, even though the schema explicitly allows it via minItems: 0.
A required group-array with min: 0 could never be submitted empty.

Changes Made

• src/validation/schema.ts

  • In the required check, an empty array is no longer automatically treated
    as a missing value when the field schema declares minItems. In that case
    the array counts as present, and its length is validated by minItems
    (which runs separately) rather than by required.
  • Behaviour is unchanged for required arrays that do not declare
    minItems: an empty array is still treated as missing, preserving existing
    behaviour.

• test/validation/required_field.test.ts

  • Added a test that a required array with minItems: 0 accepts an empty array.
  • Added a test that a required array with minItems: 1 reports a minItems
    error (not a required error) for an empty array, confirming length is
    still enforced by the right mechanism.
  • Added a test that a required array without minItems still treats an empty
    array as missing.

Related Issues/PRs

Fixes #247

Checklist

• I have read the Contributing Guidelines and followed them.
• Unit tests were added.

[spiderocious]

@sandrina-p @danielcardoso5 please review